The SIR provided L-[4'-'80]Tyrosine; Ig Quinones represent a newly discovered class of organic cofactors formed by postranslational hydroxylation of aromatic an-iino acids, thereby allowing them to servie as redox catalysts at tenzyme active sites. The trhydroxyphenylalanine cofactor, TPQ, is found in amine oxidases (AO) where it catlyzes the oxidation of organic amines to aldehydes. This reaction is initiated by amine attack at the C5 carbonyl group, followed by oxidation to an imine and subsequent release of an aldehyde. The reduced aminoquinol cofactor is reoxidezed to the quinone in two one- electron steps involving a semiquinone intermediate. This reoxidation reaction utilizes Cu2+ as an immediate electron acceptr and 02 as the terminal electron acceptor. Resonance Raman (RR) spectroscopy has proved to be an excellent technique for studying quinone cofactors in proteins. These chromophores produce strongly enhanced spectra whose vibrational pattern is unique for each type of cofactor;. In our study of the TPQ cofactor of AO during its reaction cycle we were able to identify oxidation states of the quinone and catalytic intermediates by RR spectroscoy. These studies were aided by the use of isotopes and an extensive set of synthetic models that provided a firmer structural basis for vibrational assignments. Future directions will be to elucidate the mechanissm of biosynthesis, to provide detailed information about catalytic reactivity, and to identify the structures of novel quinone cofactors in other enzymes.